Magnetoactive feature of in-situ polymerised polyaniline film developed on the surface of manganese–zinc ferrite

► In-situ grown PANI film on the surface of MnZn ferrite exhibits magnetoactive properties. ► Ferrite accelerates PANI growth by sorption of paramagnetic anilile oligomers on domain walls. ► Pinning of domain walls restricts their mobility in magnetic fields. ► Pinning forces are strong enough to induce magnetic anisotropy in low-anisotropy MnZn ferrite. ► Stress-induced magnetic anisotropy increases inner demagnetising factor of coated ferrite particles. A polyaniline film exhibits magnetoactive properties when deposited on the surface of multidomain particles of manganese–zinc ferrite during in-situ polymerisation of aniline. This is reflected in the increased coercivity and thermomagnetic stability of an in-situ prepared composite compared with bare ferrite and its mixed composite with polyaniline. In addition, the deposition of a polyaniline film results in a shift of the complex-permeability dispersion region towards ultrahigh frequency band. These changes in the magnetic properties of polyaniline-coated ferrite are attributed to the increased value of the inner demagnetisation factor, which results from stress-induced magnetic anisotropy due to the pinning of domain walls appearing on the surface of ferrite. This study is focused on the mechanism of pinning of domain walls and its influence on the magnetic properties of in-situ prepared composites in terms of the molecular mechanism of oxidative polymerisation of aniline. Ferrite stimulates the propagation of polyaniline chains, which start to grow on the domain walls on the ferrite surface. It leads to the pinning of domain walls and restricts their mobility in a magnetic field. The further increase in the coercivity and the resonance frequency of polyaniline-coated ferrite due to film shrinkage after deprotonation of polyaniline makes it obvious that polyaniline coating induces elastic stresses in a ferrite particle that stimulate the growth of the effective magnetic anisotropy. Stress-induced magnetic anisotropy contributes to the reorientation of the magnetisation vectors in domains with respect to the new directions of easy magnetisation, given by magnetoelastic stresses, which leads to complex changes in the magnetic properties of in-situ prepared composites.